I've found a few previous threads that reference this problem, but it's pretty horrific especially now LTE is in the picture.
Despite setting the "Use Cellular Data" option to OFF (verified in both Music and iTunes settings), if you select music to be downloaded (i.e. click the cloud icon), iTunes grabs the music regardless of whether you are on WiFi or Cellular.
Scenario 1: I get a new iPhone5 and want to sync my existing music library to it. I have a playlist with all 2500 songs in it, so I go there and hit the cloud icon so all my music copies over. There's no way to pause the whole process - only individual songs - and no way to stop it using cellular data, so when I leave the house, my phone starts eating up my cellular data plan (2.5GB in 3 days, when most of my time was spent at home). I detailed this on my blog - http://lamejournal.com/2012/09/24/itunes-match-uses-cellular-data-even-when-you- say-no/
Still, it shouldn't be so bad, because I spent most of my time at home. Right?
Scenario 2: Your brand new iPhone 5 has problems staying connected to WiFi (see the many threads on this issue). What happens when it has problems with WiFi and drops off the network? Yup, you got it, it uses LTE instead. So while I'm sitting in my house, 10 feet away from a 802.11n access point, supposedly 'safely' downloading my music over my home Internet connection, my phone randomly decides it doesn't want to play with WiFi after all, and continues downloading on cellular instead. Isn't that great?
This is ridiculous. The option "Use Cellular Data" should do exactly what it says - when turned OFF, iTunes should not be downloading iTunes Match data from the cloud over cellular. I have 20Gig of music to download, and thank goodness I caught it when I did before being hit with enormous overages.
Hopefully this is just a bug and can be fixed easily and quickly by Apple.